LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

2 LOWER CRETACEOUS DEPOSITS INT CALIFORNIA AND OREGON
sible to attain a more satisfactory interpretation of the faunas found in
the Lower Cretaceous deposits of the West Coast. These works have
also led to a desire to obtain more definite information as to the stratigraphical
and faunal successions in the richly fossaliferous strata of the
Knoxville and Shasta series in California and Oregon, and a persistent
effort has been made in recent years to do this. As a result large collections
of invertebrate fossils have been gathered from the lower units of
the Sbasta series and from the upper units of the Knoxville in California
and Oregon, and it is now possible to fix more definitely the line of demarcation
between them, even in areas in which stratigraphical unconformity
i3 not evident. However, it is not believed that either class of
evidence is complete, or that mapping will always be easy.
Two major stratigraphical groups have been recognized in the Shasta
series in California, both of which were foreshadowed in the work of
Diller and Stanton, although these writers did not go so far as to distinguish
them clearly. These are the Faskenta group below and the
Horsetown group above, bearing evidence of a disconformity between
them, as will be shown in the following pages.
In its general faunal aspects the Shasta series presents two distinct or
not closely related faunal assemblages corresponding to the major stratigraphic
groups, both of which are distinct from that of the Knoxville
series; The study of these assemblages reveals cogent evidence as to
important diastrophic changes of wide geographical extent supporting the
deductions derived from the stratigraphical relationships in the field.
In the Paskenta group there are many forms of Molluscs, including
Aucellas and cephalopoda, the nearest analogues of which are found in
contemporary deposits in Russia, as noted by Pavlow and earlier writers.
In fact suggestions have been made by Stanton and others as to routes
of migration, or of exchange, between Russia and western America,
The boreal character of Aucella has suggested routes of exchange by
way of the Arctic seas, notwithstanding the fact that species of this genus
have been found in lower latitudes (Mexico and India).
Students of paleontology, for whom thiB memoir is chiefly intended,
should they desire a knowledge of the West Coast faunas before knowing
their stratigraphical order, sources, and correlations, will find it in the
second part of this volume.
But doubtless, to many students of geology it would seem more logical
to consider first the deposits themselves, their stratigraphic order, the
physiographic conditions of their period, and their other aspects; and
these involve the nature of the sediments, their distribution, volume,
attitude, sources, transportation, and the many details of their deposition.
However, these aspects of the study should also be of interest to students
of paleontology.